Convertible band pass amplifier coupling



Jan, IQEQ G A DE 1,944,326) I CONVERTIBLE BAND PASS AMPLIFIER COUPLING Filed June 18, 1930 INVENTOR RENE A BRADEN ATTORNEY Patented Jan. 16, 1934 PATENT OFFICE CONVERTIBLE BAND PASS AMPLIFIER COUPLING Rene A. Braden, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 18, 1930. Serial No. 461,875

Claims.

In my copending application, Serial No. 456,- 024 filed May 2'7, 1930, I have disclosed methods of, and arrangements for, ideal reception of radio programs from near-by powerful broadcasting 1 stations, and from distant stations, the arrangements being such that in the case of local reception the radio frequency amplifiers of the receiving circuit possessed a band pass characteristic I whereby the receiver had a high degree of fidelii ty, and a moderate degree of sensitivity and selectivity, and in the case of distant reception the radio frequency amplifier was practically identical in construction and characteristics with the conventional tuned radio frequency amplifier receiver, whereby the receiver possessed a moderate degree of fidelity but had Very high sensitivity and selectivity.

Thus, a radio receiver was provided in the aforementioned application, which receiver included a convertible band pass radio frequency amplifier, the convertible amplifier generally consisting in providing at least one of the electron discharge tubes of the radio frequency amplifier with a tuned input circuit, and coupling the output circuit of the tube with a second tuned input circuit, an additional coupling path being provided between the tuned input circuit and the second tuned circuit, means being associated with the amplifier stage to selectively operate the stage as a band pass amplifier stage with the tube inoperative for local reception, and as a stage of tuned radio frequency amplification with the tube operative, but with the additional coupling path inoperative, for distant reception. Again, it was pointed out in the aforementioned copending application that any of the coupling methods suitable for band pass circuits was capable of being employed in the convertible band pass amplifier receiver. One of the coupling methods disclosed showed a combination inductive coupling and capacity coupling in circuit.

In a convertible band pass amplifier it is extremely desirable to utilize a combination of inductive and capacity coupling in so much as it is thereby possible to obtain a constant band width. It has been found inconvenient, however, to employ a coupling condenser because of the difliculty of opening this circuit completely when the amplifier is converted into two tuned stages of radio frequency amplification for distant reception.

In the present application, there is disclosed an improved method of, and arrangement for, utilizing the electron discharge tube in a convertible band pass amplifier stage as a coupling condenser. Accordingly, it is one of the main objects of my present invention to provide an improved method of, and means for, converting a band pass amplifier stage of a radio receiver for use in connection with local or distant reception, the method consisting in employing a screen grid tube as the intermediate tube in a band pass amplifier stage, and utilizing this tube as a coupling condenser for local reception, the tube being rendered operative as an amplifier, however, when the circuit is converted into a two stage radio frequency amplifier for distant reception.

Another important object of the invention is to provide in a convertible band pass amplifier receiver, including one or more stages of radio frequency amplification, a method of employing the screen grid tube of at least one of the radio frequency stages as a coupling condenser for a band pass stage which consists in inserting an impedance in the screen grid circuit, by means of which the capacity between the control grid and the anode of the said tube is increased to a desired value.

Still other objects of the invention are to improve generally the simplicity and efficiency of coupling means for convertible band pass amplifiers, and to provide a receiver employing at least one stage of convertible band pass amplification which is not only reliable in operation, but economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit organization whereby my invention may be carried into efiect.

Referring to the accompanying drawing, there is shown a convertible band pass radio frequency amplifier stage, wherein incoming signal energy, of a predetermined band of frequencies, after collection, is impressed across the terminals of the primary coil 1 of a radio frequency transformer T, it being clearly understood that the primary 1 may be located in an antenna circuit (not shown), or may even be the output circuit of a preceding stage of radio frequency amplification. The secondary coil 2 of the transformer T is tuned by a variable condenser 3, thus providing a tuned input circuit I for the electron discharge tube 4, the latter preferably being a 1 0 screen grid tube.

The anode of the tube 4 receives potential through a tuned circuit II which consists of an inductance coil 5 shunted by a variable condenser 6, the source of anode current being omitted for the sake of simplicity of showing. The amplified output of the tuned output circuit of the tube 4 is then employed in any desired manner, as for example, in a succeeding stage of radio fre quency amplification, detector stage and subsequent audio amplification followed by utilization in a loud speaker, head phones, or the like. The stages subsequent to the tuned circuit 11 are not shown in order to simplify the description, and also because such subsequent circuits are well known to those skilled in the art and need not be further described.

The screen grid element 7 of the tube 4 is positively biased relative to the cathode of the tube, and receives potential from a source (not shown) through a switch mechanism conventionally shown in the drawing, and designated by the letter S. An impedance, generally designated by the letter Z, is connected in the screen grid element circuit, the impedance being connected in shunt with the switch S. By means of the impedance inserted in the screen grid circuit the capacity between the control grid and the anode of tube 4 is increased to a desired value.

A small inductance coil 8 coupled to the inductance coil 2, a small inductance coil 9 which is coupled to the coil 5, and a coil 10, which is connected in series with the coils 8 and 9, constitute a link coupling, such as has been disclosed in my aforementioned copending application, for inductively coupling the tuned circuits I and II. A switch 11 is inserted in the link coupling circuit in order to connect the link circuit in and out of circuit with the tube circuit. If the switch S is closed and the switch 11 is opened, the tube 4 and its tuned circuits form a complete tuned radio frequency amplifier stage.

If the switch S is opened, and the switch 11 is closed, the tube 4 no longer acts as an amplifier tube, and its only eifect on the circuit is due to its inter-electrode capacities. The two tuned circuits I, II then are coupled by the link circuit 8, 9 and 10, the link circuit being adjusted as described in my copending application, Serial No. 390,468 filed September 5, 1929, so that the coupling between the circuits I, II is in the neighborhood of the critical coupling value, the tube 4 and its two coupled tuned circuits then forming a band pass radio frequency amplifier stage for the reception of local signals.

When the screen grid element 7 is grounded, that is to say, connected to ground through a conductor of no appreciable impedance, the capacity between the control grid and anode is very small, generally about 0.005 to 0.02 micro micro farads, depending on the structure of the tube 4 and the completeness of the screening action. If the screen grid element '7 is ungrounded, and is not connected to anything, that is to say, connected to ground through a conductor of infinite impedance, the capacity between the control grid and the anode is practically the same as it would be if the screen grid '7 were removed without disturbing the other elements.

If the screen grid element 7 is connected to ground through conductors having impedances intermediate between zero and infinity, then the capacity between the control grid and the anode will have values intermediate between the screened value (0.005 to 0.02 micro micro farads) and the unscreened value (5 to 10 micro micro farads). Thus, by selecting a suitable impedance to be placed between the screen grid element 7 and ground, the capacity between the control electrode and the anode can be made to take any desired value. It will be noted from the circuit shown in the drawing that this control grid-anode capacity is so connected to the two tuned circuits as to function as a coupling capacity. The switch S is employed to shortcircuit the impedance Z when converting the circuit to a two stage amplifier, that is to say, when the switch S is closed the tube 4 is rendered operative, as clearly explained heretofore. Thus, switch S performs two functions: (1) puts positive bias on screen; (2) shorts out Z so that gridplate capacity is low.

The convertible band pass amplifier stage described heretofore is thus designed to meet the requirements for both local and distant reception. For local reception, the receiver has band pass characteristics in the radio frequency circuits, and there is, therefore, no loss of fidelity (attenuation of high notes) in these circuits. The sensitivity is less than is considered essential for receiving from distant stations, but is suficient for reception from local stations. The receiver is quite selective, but not sufficiently selective to tune out a strong signal and tune in one on the adjacent wave channel. The selectivity is more than adequate for the separation of local stations which work on well-separated wave frequencies.

For reception from distant stations, the sensitivity is increased considerably, the band pass characteristic is reduced or perhaps eliminated, the selectivity with respect to closely adjacent wave channels is increased, and the fidelity is reduced. When adjusted for the reception of di tant signals, the convertible'receiver is practically identical in characteristics with the conventional tuned radio frequency receivers. Furthermore, it is possible, according to the present disclosure, to use combined inductive and capacity coupling in a convertible band pass amplifier whereby a constant band width is assured, the difiiculty of opening the coupling condenser circuit when the amplifier is converted to two tuned stages being substantially eliminated by selecting a suitable impedance to be placed between the screen grid element and ground, the capacity between the control grid and anode thus being made to take any desired value. It is, thereby, conveniently possible to employ the capacity between the control electrode and anode of the band pass tube as a coupling capacity when the tube acts in a band pass stage.

while I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. An amplifier circuit comprising in combi-, nation an electron tube having an anode, cathode control electrode and screen element, sources of voltage of suitable magnitude for the electrodes, input and output circuits, a direct current path connected to the screen element, and in addition, an impedance connected in shunt to the path so connected between the cathode and screen element that all the alternating current flowing into or out of. the screen element must pass through said impedance.

2. A radio frequency amplifier comprising a tube provided with an anode, screen grid, cathode and control grid, a tunable input circuit connected between said control grid and the cathode, means for connecting said screen grid to a source of positive potential, an output circuit for the tube, and a link circuit for coupling the input and output circuits, and an impedance connected between said screen grid and cathode for increasing the capacity between the control grid and anode to a desired value.

3. A radio frequency amplifier comprising a tube provided with an anode, screen grid, cathode and control grid, a tunable input circuit connected between said control grid and the cathode, means for connecting said screen grid to a source of positive potential, and an impedance in shunt with said connecting means connected between said screen grid and cathode for increasing the capacity between the control grid and anode to a desired value.

4. A radio frequency amplifier comprising a tube provided with an anode, screen grid, cathode and control grid, a tunable input circuit connected between said control grid and the cathode, means for connecting said screen grid to a source of positive potential, and an impedance connected between said screen grid and cathode for increasing the capacity between the control grid and anode to a desired value, said connecting means including a switch.

5. A radio frequency amplifier comprising a tube provided with an anode, screen grid, cathode and control grid, a tunable input circuit connected between said control grid and the cathode, means for connecting said screen grid to a source of positive potential, and an impedance connected between said screen grid and cathode for increasing the capacity between the control grid and anode to a desired value, a tunable output circuit being coupled between said cathode and anode, a link circuit magnetically coupling the input and output circuits, and means for rendering the screen grid connecting means and link circuit operative or inoperative at will.

6. A radio frequency amplifier comprising a tube provided with an anode, screen grid, cathode and control grid, a tunable input circuit connected between said control grid and the cathode, means for connecting said screen grid to a source of positive potential, and an impedance connected between said screen grid and cathode for increasing the capacity between the control grid and anode to a desired value, and an additional coupling path between said input circuit and said anode circuit.

7. In a convertible band pass amplifier adapted to be used in a radio receiver, an electron discharge tube provided with an anode, a control electrode and a screen element, a means for tuning the input circuit of the tube, means for tuning the output circuit of the tube, an additional coupling path between the two tuned circuits, an impedance of a predetermined value disposed in the screen element circuit, and means for short-circuiting said impedance to employ the tube as an amplifier in a stage of tuned radio frequency amplification.

.8. In a convertible band pass amplifier adapted to be used in a radio receiver, an electron discharge tube provided with an anode, a control electrode and a screen element, a means for tuning the input circuit of the tube, means for tuning the output circuit of the tube, an additional coupling path between the two tuned circuits and inductively coupled thereto, means for disconnecting said coupling path from said tuned circuits, an impedance of a predetermined value disposed in the screen element circuit, and means for short-circuiting said impedance to employ the tube as an amplifier in a stage of tuned radio frequency amplification.

9. In combination, in a band pass amplifier stage, an electron discharge tube provided with a control electrode, a screen element and an anode, a tuned circuit connected to the input circuit of the tube, a second tuned circuit connected to the output circuit of the tube, the two tuned circuits being inductively coupled, the capacity between the control electrode and anode of the tube additionally coupling the two tuned circuits, and an impedance of a predetermined value disposed in the screen element circuit whereby said capacitive coupling can be made to take any desired value.

10. A radio receiver adapted for both local and distant reception with optimum results in each type of reception, comprising at least one stage of radio frequency amplification including an electron discharge tube provided with an anode, a control electrode and a screen element, input and output circuits for the tube, means for tuning the input circuit of the tube, means for establishing a coupling path between the tuned input and output circuits, means for converting the amplification stage into a band pass stage comprising a device for rendering the tube inoperative and an independent device for simultaneously rendering the coupling path operative, the interelectrode capacity between the said control electrode and the anode of the tube being thereby employed as a coupling capacity between the said two circuits.

RENE A. BRADEN. 

